High-power light emitting diodes, microprocessors, and other compact, high heat generating electronic devices need to be mounted on a thermally conductive substrate that is ultimately thermally connected to a heat sink. The best thermal path to the heat sink is through an all-metal path. However, a polymer dielectric layer typically exists between the electronic device and the heat sink for providing electrical isolation between the device electrodes and the heat sink. To avoid the polymer dielectric layer, it is known to directly bond a copper interconnect layer to an insulating ceramic substrate (e.g., Al2O3 or AlN), where the electronic device is mounted on the copper interconnect layer, and where a small-area metalized bottom surface of the ceramic substrate is soldered to a relatively large metal plate for mechanical mounting (e.g., via bolts) to a heat sink. The large metal plate helps to spread heat laterally across the heat sink for increased heat removal and provides a means for attaching the ceramic substrate to the heat sink. Such a ceramic substrate is known as a direct bonded copper (DBC) substrate. The thermal conductivity of a thin ceramic substrate is much greater than that of a polymer layer. The metal plate is bolted to a heat sink, with a thermally conductive thermal grease between the plate and heat sink to ensure full thermal contact. A thermal grease typically is infused with metal (e.g., silver) for high thermal conductance.
In applications where there is a need for even further thermal control, the electronic device is thermally coupled to the top surface of a vapor chamber for increased spreading of heat, and the bottom surface of the vapor chamber is affixed to a heat sink. By spreading the heat, the overall thermal resistance is reduced. Vapor chambers typically provide greater 30% more heat spreading than a solid metal plate. Heat spreaders other than a vapor chamber may be used.
A vapor chamber is a thin closed metal chamber, typically formed of copper, with flat top and bottom surfaces. The chamber contains a small quantity of a working fluid, such as water, under a partial vacuum. The chamber also contains a wick. The heat source is thermally coupled to the top surface, and the bottom surface is thermally coupled to a heat sink. The heat source vaporizes the water in the chamber near the top surface to create a phase change. The vapor is then cooled at the bottom surface and turns into a liquid. This creates a pressure differential that speeds up the movement of the liquid back to the top surface by capillary action through the wick. The flowing of the liquid/vapor inside the vapor chamber helps spreads the heat in two dimensions across the vapor chamber area (in-plane spreading) and the heat is conducted in a vertical direction (through-plane) to the heat sink. By spreading the heat over a relatively large area (compared to the size of the electronic device), the thermal resistance between the electronic device and the heat sink is reduced.
Further details of vapor chambers are described in US Publication Nos. 2006/0196640, 2007/0295486, and 2008/0040925, and U.S. Pat. No. 7,098,486, all incorporated herein by reference.
It is known to affix a metal core printed circuit board (MCPCB) to the top surface of a vapor chamber, such as by a thermally conductive epoxy, but any polymer dielectric layer over the circuit board increases thermal resistance, and the epoxy is not as good of a heat conductor as metal. It is also known to directly solder LED chips to the top surface of the vapor chamber (U.S. Pat. No. 7,098,486) and use the metal of the vapor chamber as an electrode, but this technique has many drawbacks, such as requiring special equipment to connect the delicate electrodes of the LEDs to other than the standard circuit board or submount.
If the electronic devices were first mounted on a conventional circuit board or ceramic submount, and the back surface of the relatively large circuit board or submount were somehow soldered directly to the top surface of the vapor chamber, voids in the solder layer under the board or submount may develop due to the relatively large surface area being soldered. Typically, the board or submount would be soldered only around the edges. Further, a solder reflow technique (typically used for surface mount devices) could not be used since it would subject the fluid in the vapor chambers to temperatures around 230° C., which exceeds the typical maximum allowable temperature for the vapor chamber. Therefore, any soldering to the vapor chamber must be done by other than surface mounting technology solder reflow.
What is needed is an improved technique for thermally coupling a heat generating electronic device, such as one or more high power LEDs or a microprocessor, to a top surface of a vapor chamber, and thermally coupling the vapor chamber to a heat sink.